Cable Design And Development Of Insulation Materials

DESIGN AND DEVELOPMENT OF INSULATION MATERIALS FROM PAPER INSULATION TO XLPE INSULATION FOR POWER CABLES A)

Paper Insulation: The invention of basic cable technology i.e. an insulated conductor can be traced back to year 1830. It took nearly 50 years, until the first underground cable for transmission of power, was put into operations, around year 1880. The first insulation materials were gutta-percha, natural rubber, impregnated jute and somewhat later impregnated paper. Impregnated paper, as stable insulating material, could get universally accepted due to its resistance to heat and maintaining the dielectric strength for a long period of time, with the coupled advantage of low dielectric losses. Thus, started the era of development of various types of paper insulated cables, right up to 400kV, which can be described as below, as mile-stones:  Multi-layer dielectric, using lapped paper tapes.  Vacuum impregnation of dried paper insulation, using heated resinous mass (thus the name- mass impregnated cable).  Moisture proof encapsulation of insulation by seamless lead sheath.  Development of “radial electrical field” by using metalized screening over insulated cores. As the cables were being designed and developed for higher loads and higher voltages, a principle design difficulty was encountered i.e. formation of micro-voids, in the insulation due to cyclic thermal loading, during operations. Partial discharges occurred in these micro-voids under “high field strength” and damaged the insulation, leading to breakdowns. This difficulty was overcome by development of oil-filled cables, whereby oil-filled the micro-voids due to positive pressure and thus “Partial discharge free thermally stable cable” rated for 100kV was commissioned in Italy in 1924. Thus started the era of oil-filled EHV cables. 1/7

Over the time the paper insulation technology was further refined, leading to external gas pressure cables. Thus, since early 70s, we have oil-filled and gas pressure cables, with paper as the basic insulating material, up to 400kV, under commercial operations, all around the world. B)

Thermo Plastic Synthetic Insulation Materials: While the above developments in the progress of paper insulated cables for higher voltages was taking place, there was always a desire to have single layer dielectric, which did not involve the process of impregnation, which is very slow and not so neat. Development of synthetic plastics (polymers) coupled with the technique of continuous process of extrusion of insulation over the conductor, could thus replace the paper wrappings, impregnation process, handling of the fluid components (oil or gas) and the difficulties associated with the same. The first precursor of the present day polymer insulated cables i.e. Poly-Vinyl Chloride (PVC) Cables, were first manufactured in mid 1940s. This material could get accepted for low voltage application, however, was found to be unacceptable for high voltage application, due to high dielectric losses. Thus, PVC, which is the most popular insulation material for low voltage application, does not find acceptance beyond 11kV for commercial operations. The decisive milestone in the synthetic high voltage cables was reached with the development of thermo-plastic polyethylene (PE), which has excellent low dielectric losses matching that of the paper insulation. The first 3kV PE cable was used in USA in 1944 and 20kV PE cable in Switzerland in 1947. Ethylene, with chemical formula C2H4 has entirely symmetrical structure and can be polymerized (long chains of carbon and hydrogen) with heat and pressure alone. Thus, we have a polymer of ethylene, called polyethylene. PE is a thermo-plastic material like PVC and has a softening temperature of 1100 C to 1300 C. 2/7

However, this material being thermo-plastic, i.e. becoming soft when heated (due to I2R losses) is not suitable for operating a cable conductor at temperatures higher than 600 C (conductor temperature of PVC cables is 700 C). On application of heat, the molecular chains of PE slide over each other, and the material becomes soft, similar to PVC behavior. C)

Thermo-set (Cross-linked) Insulation Materials: Dow Corning, in early 60s developed a technique, whereby with the application of heat, the relative layers of molecular chains of PE were prevented from sliding over each other, thus, thermo-set PE was developed, which dose not become soft, with the application of heat. The technique is to break the molecular chains of some of the molecules of PE, by taking away the hydrogen atoms, which leads to direct link between the carbon atoms of adjacent molecular chains. The material dose not become soft or melt with the application of heat and thus, is “thermo-set” material or crosslinked polyethylene i.e. XLPE . In simple form, the three stages can be shown as under: a)

∼ CH2 ∼ CH2

CH2 CH2

CH2 CH2 ∼ CH2 CH2 ∼

Molecular Chains of Polyethylene (Thermo-plastic material) PE

b)

∼ CH2

CH • • CH

CH2 CH2 ∼

Removal of Hydrogen atom From two molecules (Intermediate stage)

CH I CH

CH2 CH2 ∼

∼ CH2 c)

∼ CH2 ∼ CH2

D)

CH2 CH2 ∼

CH2 CH2 ∼

Joining of carbon atoms of of adjacent molecular layers (Thermo-set material) XLPE

Development of techniques of Cross-linking of Polyethylene: Over the years, three basic techniques of cross-linking PE have been developed as under: 3/7

a) i)

Addition of peroxide Cross-linking with high pressure steam This is a conventional cross-linking (or vulcanizing or curing) technique. A compound of peroxide is added to PE by the PE manufacturer. The material is extruded over conductor like any thermo-plastic material. The crosslinking reaction needs high temperature and pressure. Therefore, the extruded core is moved through a hollow tube called Continuous Catenary Vulcanizing line (CCV). The tube has super heated steam at 2000 C with the pressure of 15 to 20 bars. These cables are called “Steam-cured cables” and are being produced up to 66kV,(in some cases up to 132 kV) since late 70's. The stress level in the insulation for such cables is in the order of 3.5kV to 5kV per mm. The production is very fast and economical due to outstanding heat transfer property of steam when super heated. However, it may be noted that the moisture defuses into the hot mass of molten synthetic material, and affects the longevity of the cable.

ii)

Gas Cured cables or dry cured cables: For 110kV to 150kV cables, where the stress level is 6.5kV to 7.5kV per mm, the steam is not used as medium of heat transfer and therefore, nitrogen gas is used, as the heat transfer medium and therefore, cables produced by such technique are called dry-cured cables, since there is no moisture present, during curing of XLPE insulation.

iii)

Horizontal and Vertical Curing Techniques: For 220kV & 400kV XLPE Insulations, the thickness of insulation is very high, over 20 mm for 220kV cables and over 25 mm for 400kV cables, the normal Catenary curing technique can not be employed since the same leads to eccentricity in the insulation. Further, the stress level is also very high of the order of 7.5 to 8.5kV for 220kV cables and 11 to 12.5 kV per mm for 400kV cables. Therefore, very specialized curing machines are used for the production of such Extra high voltage cables. The horizontal curing technique is called MDCV process developed by Mitsubishi and Dainichi - Nippon, Japan.

4/7 The vertical line is also called tower line or vertical continuous vulcanization (VCV) system. b)

Cross-linking by Silane process (Sioplas Technique): In this technique, suitable "alkoxy-silanes" are radically "grafted" into polymer chains of PE. This process is called condensation process and cross-linking takes place in the presence of water. In this technique, the PE material is mixed with alkoxysilanes, in a hopper at the cable works, heat is applied which starts the chemical reaction of cross-linking. While the chemical reaction is in progress, the material is extruded over the cable conductor and the extruded conductor is kept in hot water or in steam chamber for long duration, to complete the cross-linking reaction. Because the mixing of the two materials takes place in the hopper of the extruder in the cable works, “the fine process controls” cannot be ensured. This technique of production of cross-linked polyethylene was developed in early 80s and is being used extensively for 1.1kV XLPE Cables. The technique has become popular due to reduction in overall cost of the cables, since wastages are limited, which are very high in CCV line, in the form of trial-lengths, till the production process stabilizes. For last few years, the technique has been accepted for 11kV and in some cases, for 33 kV XLPE insulations, as well.

c)

Cross-linking by Electron Beams: The PE polymer chains can be cross-linked directly by means of high-energy electron beams without the necessity of heating stage, which is essential in both the above techniques. This technique can however be used for thin cable insulations and needs very high capital expenditure for installation of electron beam equipment and also the associated expenses for the safety of the personnel and environment.

This technique is very popular for production of heat shrinking tubes and components, which are to be crosslinked before expansion to higher diameters and cooling. 5/7 E)

Triple Extrusion Process: Because of the high sensitivity of XLPE insulation to partial discharge, a reliable well adhesive, and cavity free bonding of conductive layers, is of great significance for long life expectancy of the cable. While the XLPE cables were being developed for higher voltages in the early 60s, the “triple extrusion process” was developed, whereby permanently strong with homogeneous bonding, with maximum cleanliness, semi-conducting layers, were provided on either side of the insulation i.e. between the conductor and insulation on one hand and on the top of the insulation on the other hand. This firm bond between the three layers of synthetic materials acts as closed system and offers the special advantages whereby no dust or other foreign particles are able to penetrate the sensitive interfaces with the XLPE insulation. In this process the inner semi conducting layer, XLPE insulation and the outer semi-conducting layer are extruded in one single operation, using the so-called triple extrusion head, on to the cable conductor. The resistance of the semi-conducting layers is sufficient to control the very small partial charge and discharge currents over small distances i.e. over the circumference of the core. For the transmission of these currents in the longitudinal direction of the cable, towards the earthed point, additional element having substantially lower specific resistance in the form of copper tape or screen is employed. The triple extrusion, in combination with copper screen, keeps the electrostatic field in the radial direction, thereby enhancing the life of the cable and also reducing the core insulation thickness. The thermal characteristics of both the semi-conducting layers have to be compatible with the insulation of the cable, so as to provide long maintenance free performance of the cable. Any imperfection in this technique would adversely affect the cable longevity.

The thickness of the semi-conducting layers both for conductor and core is in the range of 0.1 mm onwards depending upon the voltage of the cable. For 110kV cable the thickness is 1 mm and for 230kV cables the thickness is 1.5 mm. 6/7 Thus, the performance of the cable, against partial discharges, and in-turn, longevity of the cable, depends upon how precise and perfect is the technique, to provide the triple extrusion system in the cable. F)

Influence of material purity, interface homogeneity: Perfections in the development of cross-linking techniques and also the compatible triple extrusion techniques, over the years, have made XLPE cables very economical and reliable and also almost maintenance free up to 400kV. This development would have been inconceivable without the steady improvement in the purity of the materials and homogeneity of the interfaces between insulation and semi-conducting layers. Extra High Voltage manufacturers thus, classify the insulating compounds for XLPE cables into three classes of purity, for different rated voltages, as under:  Super clean compounds

for cables up to 110kV

 Extra clean compounds

for cables up to 275kV

 Ultra clean compounds

for cables up to 500kV

Therefore, the XLPE cables ,with low dielectric losses comparable with the paper insulation, have thus replaced, the paper-insulated oil filled cables and gas pressure cables, almost entirely, for new installations, up to 400kV. ******* 7/7